In this current year, we have studied the juxtaglomerular cells in patients with renal hypertension secondary to: (1) Unilateral renal hypoplasia. In this child, high plasma renin levels could be correlated with large numbers of juxtaglomerular cells. They were seen adjacent to and sometimes in the interior of hyalinized glomeruli or in loose nests scattered in the interstitium. It appears that the juxtaglomerular cells are able to produce and release renin independent of the structural integrity of the juxtaglomerular apparatus. (2) Juxtaglomerular cell tumor. This fifteen-year-old girl also presented with high renin hypertension. An encapsulated small juxtaglomerular cell tumor, found to be the cause of hypertension, was studied by light and electron microscopy. It has the histologic features of a hemangiopericytoma or glomangioma. The presence of sympathetic direct nerve terminals on the juxtaglomerular cells was observed. This finding underscores the close biologic relationship between the sympathetic and renin systems.